Signal-precorrecting apparatus for minimizing quadrature distortion



Apri 5, i966 D. RICHMAN -PRECORRECTING APPARATUS FOR' SIGNAL MINIMIZINGQUADRATURE DISTORTION Orlglnal Filed Jan. 24, 1957 5 Sheetsw-Sheec 1 .DEme Ezmm who o 205mm nzqmmm m migo@ mNO Aprll 5, 1966 D. RICHMAN-PREGORRECTING APPARATUS FOR MINIMIZIN Original Filed Jan. 24, 1957SIGNAL G QUADRATURE DISTORTION 5 Sheets-Sheet 2 April 5, 1966 D. RICHMANSIGNAL-PREC ING ORRECT APPARATUS FOR G QUADRATURE DISTORTION 3Sheets-Sheet 3 MINIMIZIN Original Filed Jan. 24, 1957 5G mem 2 m UnitedStates Patent O illinois Continuation of application Ser. No. 636,198,Jan. 24, 1957. This application .lune 9, 1961, Ser. No. 116,210

11 Claims. (Cl. 178-6) GENERAL This invention relates toSignal-precorrecting apparatus and, particularly, to such apparatus foruse in the transmitter of a vestigial-side-band communicaitons system toprecorrect the transmitted signal for quadrature distortion inherent toenvelope detection of the single-sideband portion of the transmittedsignal in a receiver.

This application is a continuation of application Serial No. 636,198,-led January 24, 1957, and entitled, Signal- `Precorrecting Apparatusfor Minimizing Quadrature Distortion.7

One of the most common types of vestigial-side-'band communicationssystem is the present-day television system. In such systems,vestgial-side-'band transmission is utilized in order to save spectrumband width. To this end, both lower and upper side bands are transmittedfor only the low-frequency modulation components of the message orintelligence being transmitted. Only one set of side bands of the higherfrequencymessage components is transmitted. By not transmitting theother set of side bands of these higher frequency components, the amountof signal spectrum required for the total signal is reduced.

As is widely known, envelope detection by, for example, a simple diodedetector circuit of signal components transmitted in a single-side-.bandmanner produces so-called quadrature distortion of the detected signal.Such distortion arises from the presence in the detected signal ofundesired additional single-side-band components which are in phasequadrature, that is, 90 out of phase,with the received carrier signal.In a television system, such undesired quadrature components areparticularly bothersome because, in addition to distorting the detectedsignal, they serve to increase the apparent carrier level. In a negativemodulation system, like the present-day television system, such apparentincrease in carrier level causes a suppression of the brightness levelof the corresponding portion of the reproduced image.

Many schemes have been heretofore proposed for minimizing the quadraturedistortion caused by envelope deI tection of the single-side-bandportion of a transmitted signal. Some of these schemes involvetransmitter modifications, others involve receiver modifications, whilesome involve both transmitter and receiver modifications. Consideringspecically the case of television systems, large numbers of televisionreceivers of conventional design are already in existence. It would,therefore, be most desirable to have some means for correcting thequadrature distortion whereby such correction can be carried out whollyat the transmitter. This would improve the quality of the reproducedimage without additional expense to existing receiver owners. Inaddition, it would be desirable to lhave a flexible technique forminimizing quadrature distortion which would lbe readily applicable toimproved types of receivers having improved signal-handlingcharacteristics.

The schemes heretofore proposed for minimizing er1- velope detectorquadrature distortion by means of a correction which is applied at thetransmitter have generally involved a judicious choice offrequency-versus-transmission pass-band characteristics such that thebest compromise between quadrature distortion and direct distortion ofthe desired in-phase components is obtained. 'In general, such schemeshave been limited to modification of only the lower frequency-modulationcomponents which are transmitted in a partially double-side-band manner.It would, however, be desirable to have a method of correcting suchdistortion over substantially the entire spectrum of the transmittedsignal. Also, it would be desirable to have a more complete form ofcorrection not involving compromisebetween the different forms ofdistortion. In the case of a television system, the previously proposedschemes have been generally limited to use with receivers havingparticular specic types of signal-translating characteristics which aredifferent from those of present-day television receivers and, hence, notreadily usable with such present-day receivers.

It is an object of the invention, therefore, to provide new and improvedapparatus for minimizing quadrature distortion which avoids one or moreof the foregoing limitations of such apparatus heretofore proposed.

It is another object of the invention to provide new and improvedsignal-precorrecting apparatus for use in the transmitter of avestigial-side-band communications system for precorrecting envelopedetector quadrature distortion over substantially the Whole range ofsignal components which are transmitted in a purely single-side-bandmanner.

It is a further object of the invention to provide new and improvedsignal-precorrecting apparatus for use in the transmitter of avestigial-side-.band communications system whereby envelope detectorquadrature distortion occurring in a receiver may be minimized by meansof equipment modifications required only at the transmitter.

It is an additional object of the invention to provide new and improvedsignal-precorrecting apparatus for use in a television transmitter forminimizing the visible effects of envelope detector quadraturedistortion in the reproduced images of existing television receiverswithout any added expense to the receiver owner. l

In obtaining the desired signal precorrection, it appears desirable toimpart a certain phase shift .to .a relatively wide band video signalwhich, in the present case, represents part of the intelligence orinformation which is to be transmitted. Phase-shifting circuitsheretofore proposed for shifting the phase of an electrical signal aregenerally of relatively limited band width and, hence, are not suitablefor use with a wide band video signal. Accordingly, the presentinvention also relates to a new and improved wide band phase shifter andit is an additional object of the invention to provide such a new andimproved phase shifter.

In' accordance with one feature of the invention, a communicationssys-tem comprises .a transmitter for transmitting an amplitude-modulatedcarrier signal including an intelligence-signal component, part of whichis transmitted in a single-side-band manner and a receiver including anenvelope detector for detecting the intelligencesignal component, suchenvelope detection of the singleside-band portion of theintelligence-signal component causing quadrature distortion of thedetected signal. The system additionally includes means included in thetransmitter comprising circuit means for `deriving a signa-lrepresentative of the quadrature distortion, and circuit meansresponsive to the representative signal for modifying the intelligencesignal prior to encoding on the carrier signal to precorrect for suchquadrature distortion.

In Aaccordance with another feature of the invention,signal-precorrecting apparatus for use in the transmitter of acommunications system where at least a part of an intelligence signal istransmitted in a single-side-band manner comprises circuit means forsupplying an intelligence signal and circuit means for deriving a signalfor generating a quadrature signal representative of envelope detectorquadrature distortion. The invention additionally includes circuit meansresponsive to the representative signal for modifying the intelligencesignal prior to encoding on a carrier signal to precorrect forquadrature distortion caused by envelope detection of thesingleside-band portion of the transmitted intelligence signal in areceiver.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

Referring to the drawings:

FIG. l is a circuit diagram of 4a representative embodiment of atelevision transmitter including signal-precorrecting apparatusconstructed in accordance with the .present invention;

FIG. 2 is a graph representing the amplitude-versusfrequencycharacteristic of the transmitter;

FIG. 3 is a circuit diagram of a representative embodiment of atelevision receiver;

FIG. 4 is a vector diagram used in explaining the operation vof thereceiver of FIG. 3;

FIG. 5 is -a circuit diagram of a representative ernfbodirnent ofsignal-precorrecting apparatus constructed in accordance with thepresent invention;

FIG. 6 is a circuit diagram of another embodiment ofsignal-precorrecting apparatus constructed in accordance with thepresent invention, and

FIG. 7 is a circuit diagram of .a wide band phase shifter which may beused in the present invention.

FIG. 1 .-Transmiter The present invention shall be particularlydescribed for the case where it is used in a television system, thereason being that a television system is such a common and importan-tform of vestigial-side-band communications system in present-day use.Referring to FIG. l of the drawings, there is shown a transmitter fortransmitting an amplitude-modulated carrier signal including anintelligence-signal component, part of which is transmitted -in asingle-side-band manner. More particularly, the transmitter of FIG. 1represents a television transmitter, in which case the intelligencesignal is the monochrome or black-and-whte picture signal which istransmitted by such transmitter.

The transmitter of FIG. l includes a camera 10 for developing anelectrical signal representative of the scene being televised and agamma corrector 11 for precorrecting the developed signal in the usualmanner to compensate for the nonlinear characteristics of theimage-reproducing device or picture tube in the receiver. Suchgamma-corrected signal is then supplied by way of a monochrome-signalamplifier 12, a monochrome-signal corrector 13 which is constructed inaccordance with the present invention, and a signal-combining system 14to a radio-frequency transmitter 15. The operation of themonochrome-signal corrector will be ignored for the present. Alsosupplied to the signal-combining system 14 are the usual line-defiectionand field-deflection synchronizing pulses which are developed by synccircuits 16. The signal-combining system 14 is effective to develop thecomposite video signal which is then encoded `onto the radio-frequencycarrier by the radio-frequency transmitter 15. The resultingamplitude-modulated carrier signal is supplied to a vestigial-side-bandfilter 17 and then to an antenna system 18, 19 whereby it is radiatedtowards the neighboring receivers.

The effect of the vestigial-side-band filter 17 may be seen by referringto FIG. 2 which is a graph representing the amplitude-versus-frequencycharacteristic of the transmitter. For convenience, the picture carrierfrequency is taken as the zero reference frequency to which thefrequencies of the side-band components are referenced. As thereindicated, side-band components of the amplitudemodulated carrier signallying within approximately 0.75 megacycle of the picture carrier aretransmitted in the conventional double-side-band manner. Thevestigialside-band lter 17 is effective to suppress the lower frequencyset of side-band components which is .further removed from the picturecarrier than 0.75 megacycle. As a result, the video-frequencyinformation lying in the 0.75-4.5 megacycle range is transmitted in asingle-sideband manner or, in other words, only the upper set of sidebands of such video information is transmitted. It is primarily thesingle-side-band transmission of these 0.75-4.5 megacycle videocomponents which gives rise to the undesired quadrature distortion inthe receiver and it is these components which will be precorrected bythe signal-precorrecting apparatus represented by the monochrome-signalcorrector 13, the operation of which will be mentioned in detailhereinafter.

FIG. 3.-Receiver Referring now to FIG. 3 of the drawings, there is showna representative embodiment of a typical presentday black-and-whitetelevision receiver. Such receiver includes an antenna system 20, 21 forintercepting the transmitted signal which, in turn, is supplied to aradiofrequency amplifier 22 and a frequency converter 23. rfhe frequencyconverter 23, which may include the usual modulator and local oscillatorcircuits, is effective to change the signal frequency down to anintermediatefrequency value and the resulting intermediate-frequencyamplitude-modulated carrier signal is then amplified by anintermediate-frequency amplifier 24. The intermediate-frequency signalis then supplied to a second detector Z5 which is an envelope detectorsuch as, for example, a conventional diode detector and which serves toseparate the video-frequency modulation components from theintermediate-frequency carrier signal. The resulting composite videosignal is then supplied by way of a monochrome-signal or video amplifierl2.6 and directcurrent restoring means 27 to an image-reproducing deviceor picture tube 28. The deflection synchronizing components of thecomposite video signal are supplied to a deflection system 29 which, inresponse thereto, is effective to develop the usual scanning currentswhich, in turn, are supplied to defiection windings 3i) and 31 forcausing the usual deection of the electron beam of the picture tube 2Sto form the desired raster pattern. While a black-and-white televisionreceiver is shown, it is apparent that the FiG. 3 receiver could also beused to represent the monochrome-signal portion of a colorteievisionreceiver,

The effect of envelope detection by the second detector Z5 on thesingle-side-band components of the transmitted signal may be seen byreferring to the vector diagram of FIG. 4. The vectors of FIG. 4 denoteintermediate-frequency components at the input of the second detectorZS. The carrier component is represented by vector 35. The detection ofthe dou-ble-side-band (0-0.75 mc.) components is essentially inaccordance with conventional detector theory. The single-side-bandmodulation components, however, are continually varying in phaserelative to the carrier phase as indicated, for example, by the vector36 which represents one of these components. If the component denoted byvector 36 represented a sine-wave modulation component and had beentransmitted in a double-side-band manner, then another vector rotatingin the opposite direction would also be present and would represent theother side band of such cornponent. In that case, the phase of theresultant of the two side-band vectors would always correspond to` thecarrier phase and no variations would be produced along the quadratureaxis. The envelope detector 25 which responds to the resultant or peakvalue of the intermediatefrequency components would then produceanoutput signal which is sinusoidal in wave form. Suppression of one ofthe side bands, however, results in only the one side-band vector,represented by vector 36, and, accordingly, the phase of the resultantsignal varies as indicated by vector 37. A graph representing theamplitude variation with time of the resultant vector 37 would show thatthe resulting output signal is not sinusoidal in wave form but rather isasymmetrical in shape relative to the peak carrier level. Thecorresponding distortion is termed quadrature distortion because itarises from the quadrature component or quadrature etfect of thesingle-side-band signal. In addition, the asymmetrical nature of theresultant signal .causes an apparent increase in the peak carrier levelwhich, as mentioned, tends to suppress the brightness or luminance levelof the corresponding portion of the image reproduced on the face of thepicture tube 28.

MATHEMATICAL DERIVATION OF REQUIRED SIGNAL `PRECORRECTION In accordancewith the present invention, the quadrature distortion arising fromenvelope detection of the single-side-'band portion of the transmittedsignal is to be precorrected for at the transmitted by themonochrome-signal corrector 13. To this end, the monochrome-signalcorrector 13 contains the requisite circuits for performing the desiredmodilication of the conventional monochrome signal Y supplied thereto.Before the precorrection can be performed, however, it is necessary toknow the nature of the modiiication which is required. Accordingly,there shall now be given a mathematical derivation which derives anequation which describes the modification required of the conventionalmonochrome signal Y to produce the properly precorrected signal M.

The desired relationship between the conventional or present practicemonochrome signal Y' developed at the transmitter and the correspondingreproduced picture brightness or luminance displayed on the face of thepicture tube 28 in the receiver may be described by the followingmathematical expression:

where:

Y=gammacorrected present practice monochrome signal Lm=luminacereproduced by picture tube in receiver in response to the monochromesignal.

The prime symbol of the Y signal denotes that such signal has beengamma-corrected. In the case of a monochrome transmitter, suchcorrection is obtained by rising the signal to the l/'y power where 'yrepresents the nonlinearity of the picture tube 28 in the receiver. Formost present-day picture tubes, y is approximately equal to 2. Thesquare-law relation of Equation 1 assumes a ,y In order to determinelhow far the actual relationship between the conventional monochromesignal `and the reproduced luminance departs from the desiredrelationship, it is necessary to derive an expression which takes intoaccount the translation of the signal through the vari ous circuitsbetween the transmitter Iand the receiver picture tube 28. To this end,it is convenient to start with the signal Et lappearing at the output ofthe radio-frequen cy transmitter 15 and which may be defined by thefollowing expression:

Et=(1-E) cos wet (2) where:

E=video modulation wc==angnlar frequency of lcarrier t=time.

6 Equation 2 is the conventional way of expressing a modulated carriersignal with the exception that the minus sign denotes the negativemodulation technique used in television transmission.

The video modulation E may be represented by the following expression:

E=uo+2uk eos wkt (3) whe re z uo=portion of video-signal modulationtransmitted in double-side-band manner (0.075 megacycle portion ofsignal) uk cos wkt=a component of the video-signal modulation ywhich istransmitted in a single-side-band manner.

The summation sign Vdenotes that this term represents the sum of all thecomponents which are to be transmitted in a single-side-band manner.This relationship of Equation 3 may then be substituted into Equation 2and the 2 term expanded to contain terms representative of both upperand lower side bands. The lower side-band -tenms are then omittedbecause such side bands are suppressed by the vestigial-side-band -ilter17. The resultant signal is then'transmitted to the receiver wherein itis amplified by the radio-frequency amplier 22 and then supplied to thefrequency converter 23 which is effective to convert the frequency scaledown to an intermediate-frequency range. In other words, the output ofthe frequency converter 23 is similar to the transmitted signal exceptthat the frequency scale has been changed to the intermediatefrequencyvalue. This signal is then amplified by the intermediate-frequencyamplifier 24 to signal Ef which pression:

produce an output may be represented by the following ex- Ef: (l-uo) coswft-2Zkuk cos (wft-wk) (4) Where wf:intermediate-frequency value ofcarrier angular frequency Zk=receiver frequency-response or selectivityfactor.

Equation 4 corresponds to the equation for the signal which istransmitted by the transmitter except that the carrier-frequency termswf are in terms of the intermediate-frequency carrier value and thefactor Zk, representing the frequency-response factor of the receiverparticularly the ntermediatefrequency amplifier 24, has been introduced.'Ilhis factor Zk represents the gain factor of, for example, theintenmediate-frequency amplifier 24 as a function of the individualfrequency components making up t-he single-sideband portion of thesignal. In most present-,day receivers, the pass band of the[intermediatefrequency ampliiier 24 is relatively flat over thesingleside-band region in which case the frequency-response factor Zk isa constant. Such factor is, nevertheless, included in the derivation topreserve the `generality thereof so that the results Will be equally as4applicable t-o other and, perhaps, improved forms of receiverfrequency-response characteristics.

By trignometric identity, Equation 4 may be expanded to the `followingform:

Ef: (l-u0) COS wglf-lzkuk 00S wk-COS wrt-h ZZkuk cos (okt-Q00) -sin wir](5) wherein the single-side-band term is separated into an in-phaselcosine term and a quadrature-phase sine term.

To simplify the derivation, the followingl definitions will -now bemade:

Equation 8 represents the signal supplied to the second detector 25which is an envelope detector having the form of, for example, a simplediode detector circuit. In other words, the detector 25 responds only tothe resultant modulation fluctuations of the signal supplied thereto. Asrepresented by Equation 8, such input signal contains in-phase andquadrature-phase signal components and, hence, the output of detector25, which is denoted by the symbol Ed, may be represented by thefollowing expression which represents the vector sum of the inputcomponents:

11pm-mm n Equation 9 may be re-expressed as follows:

edm/1 +1) 10) where y is used to represent all the u terms as follows:

Equation l() may now be expanded in terms of a-power series as follows:

liz Lil 5 This power series may be evaluated by determining the squares,cubes, etc. of the y term represented by Equation 11. Such higher powery terms are as follows:

-4(uO-|-u) (uX) 2-i-higher power terms (13) y3=12(u0}u)2(ux)2lhigherpower terms (14) y4=higher power terms (15) Now, substituting therelations of Equations l1, 13, 14, and 15 in Equation 12, Equation 12may then be expressed as follows:

Equation 16 represents the detected signal Ed in terms of a powerseries. Such signal is then translated by a video amplifier representedby the monochrome-signal amplier 26. Accordingly, to obtain a rst ordersolution, the signal appearing at the output of the monochrome-signalamplifier 26 may be represented by the following expression:

where the terms higher than the second power have been ignored and thesquare brackets have been included to denote the band-width limitingoccurring in the amplier 26.

The derivation up to this point has been taken with the zero carrierlevel as the reference level from which signal amplitude variations aremeasured. This reference level is now changed by the action of thedirectcurrent restorer 27 which serves to establish the black level(blanking level) as the reference level from which amplitude variationsare measured. This change inreference level is now changed by the actionof the direct-current restorer 27 which serves to establish the blacklevel (blanking level) as the reference level from which amplitudevariations are measured. This change in reference level may be denotedby the following expression:

wherein S denotes the signal appearing at the output of thedirect-current restorer 227.V Because of the squarelaw nature of thepicture tube 28, this expression is more useful in its squared formwhich is as follows:

The equation may now be expressed in terms of the modulation components,u, uo, and uX by substituting Equation 17 into Equation 19 the result ofwhich is represented by the following expression:

Up to this point, it has been assumed that the video modulationrepresented by the Le terms could vary over the entire carrier-levelrange from the zero carrier level up to the blanking level (0.75 peakcarrier level). This, however, is not actually the case because thecurrent tele-l vision-signal standards restrict the video amplitudevariations to the range from reference white (0.125 peak carrier level)to the blanking level. Accordingly, in order to preserve the normalizedform of the equations such that 100 percent amplitude is equal to unity,it is necessary to change the scale factors if we Wish to speakcorrectly in terms of video-signal amplitudes as opposed tocarrier-modulation amplitudes. The necessary conversion factor isdenoted by the following expression:

Video Range =Blankingto-White 62.5 5 Modulation Range Blanking-to-Zeron* (21) In other words, assuming a percent video amplitude variation,then, in terms of the carrier-modulation range, this corresponds to avariation of ve-sixths times 100 percent or 83 percent of the carrierrange from blanking level to zero carrier level. Accordingly, the signalS in terms of carrier-modulation amplitude at the input of the picturetube 28 may be expressed in terms of video amplitude as follows:

S=5/6V (22) where:

V=video signal at picture tube 28 (0-4.2 megacycles). Similarly, itfollows that the following relations al-so hold:

s2= 5/6)2V2=(5/5)2Lm (zsy where the actual signal V is used in place ofthe de- Y sired signal Y in Equation l and where M0=double-sidebandportion of monochrome video signal (0.75-4.2 megacycles) and where videoAlso, thetotal transmitted monochrome signal M may be expressed as thesum of the double-side-band and single-side-band video components asfollows:

Substituting the relationships of Equations 23-26, inclusive, intoEquation 20 and simplifying by the relationship of Equation 27, thenEquation 2() becomes:

Equation 28 expresses the relationship between the transmitter video.signal M and the corresponding luminance L,m reproduced on the picturetube 28 of the receiver. As mentioned in connection with Equation l, itis desired that the relationship between the unmodified conventionalmonochrome video signal Y and the reproduced pi-cture luminance Lm be asfollows:

The expression of Equation 30 gives the final form of the derivation inthe sense that it describes the relationship between the conventionaluncorrected monochrome signal Y' and the monochrome signal M which isactually transmitted. In order to know how to modify the uncorrectedmonochrome signal Y', it is necessary tosolve Equation 30 for M.

One approximate solution of Equation 30 may be obtained by defining M asfollows:

where b-:additional component necessary to correct present practice (Y)signal.

This expression for M may then be substituted into the right-hand termsof Equation 3() containing the quadrature terms MX. Then, these-latterterms may be expanded and the resulting terms `containing the correctionfactor b may -be omitted because both the correction factor b and thesquare of the quadrature terms MX are small so that their product isrelatively minor compared to the other terms. When this is done, theresulting equation may then be solved for M and the solution is asfollows:

Equation 32 gives us one for-m of modification required of theuncorrected monochrome signal Y' which is necessary to produce thedesired precorrected monochrome signal M. As will be noted, theessential feature is that the uncorrected monochrome signal Y' must bemodified in accordance with certain fractions of the quadrature term MXwhich term, in turn, represents the envelope detector quadraturedistortion which will subsequently be encountered in the receiver. Thenon linear form of the equation, namely the square and square roots,results `from the nonlinear nature of the picture tube 28 in thereceiver.

Equation 32 may be stated in a sirnplied form by dening the differentproportions of the quadrature ter-ms as follows:

These definitions enable the expression of Equation 32 to be simplifiedto the following form:

Mex/m (34) As will be mentioned more in deaail hereinafter, this form ofcorrection as represented by Equation 34 may be obtained iby utilizingthe apparatus described in FIG. 5.

A more exact form `of solution of Equation 30 may be obtained byutilizing the usual quadratic solution. More specicaliy, Equation 3G maybe simplihed by making the following definition:

may be obtained by use of the apparatus of FIG. 6 which will bedescribed hereinafter.

Description of FIG. 5.-SignrzI-precorrectng apparatus Referring now toFIG. 5 of the drawings, there is shown signal-prercorrectinlg apparatusfor use in a television transmitter where at least a monochrome-signalcomponent is partially transmitted in a single-side-band manner. Morespecifically, the apparatus of FIG. 5 represents in detail one form ofapparatus that may =be used as the monochrome-signal corrector 13 of theFIG. 1 transmitter. To this end, the apparatus of FIG. 5 includescircuit means ffor supplying a monochrome signal which circuit means maybe represented by the input terminal 40 as well as the other portions ofthe transmitter which are coupled to this terminal 4t) for supplying aconventional monochrome signal Y thereto.

The apparatus of FIG. 5 also includes circuit means responsive to thesingle-side-band portion of the monochrome video signal Y for generatinga quadrature signal B fx representative of envelope detector quadraturedistortion. This circuit means may be termed a quadrature-signalgenerator and is represented by the units within the dashed line box 41.In the ycase of a television system, the quadrature-signal generator 41preferably nciudes nonlinear circuitry having a nonlinearitycorrespending to the nonlinearity of the receiver imagereproducingdevice or picture tube. More specifically, the quadrature-signalgenerator 41 may include a bandwidth limiting filter d2, a phase shifterd3, and a squaring circuit 1A. The phase shifter 43 is preferably a 90phase shifter for generating a quadrature-phased replica yof Uhesingle-side-band portion of the input monochrome signal. Because suchsingle-sideband portion is relatively wide band in nature, a novel typeof phase shifter, as will be described more particularly in connectionwith FIG. 7, may fbe used as the phase shifter 43. The hand-widthlimiting filter d2 may be of the band-pass type, as indicated, or,instead, might be of the highpass type. Squaring icircuits, such as thesquaring circuit 44, are relatively widely known in the art and may takeany one of several forms. Por example, such squaring circuit mightinclude a modulator tube wherein the signal to be squared is suppliedyto two different control electrodes thereof so that, in effect, suchsignal is multiplied by itself to produce the desired squaring Iaction.

The apparatus of FIG. 5 also includes circuit means responsive to thequadrature signal for modifying the single-side-band portion of themonochrome video signal Y to precorrect for quadrature distortion causedby envelope detection of the single-side-band portion of the transmittedmonochrome signal in a receiver. Such circuit means is representedgenerally by the remainder of` the units of the FIG. apparatus. Moreparticularly, such circuit [means may include a squaring circuit 45,modulator lcircuit means, and a square-root circuit 46 `for translatingthe monochrome video signal. The modulator circuit means may include apair of modulators 47 land 48 and an adding` `circuit 49. Thesquare-root circuit 46 may, for example, take the form of any of thewell-known types of gamma-corrector circuits which are well known in theart.

. The circuit means responsive to the quadrature signal for modifyingthe monochrome video signal also includes matrixing circuit means forproportioning the quadrature signal and -for supplying proportionedquadrature-signal components to the modulator circuit means 47 and 48for modifying the single-side-band portion of the monochrome signal toobtain the desired precorrection for quadrature distortion. Morespecifically, ysuch matrixing circuit means may include a signalattenu'ator Si) and a direct-current restorer 51.

Where the Isignal precorrection is to be applied at video frequencies,as represented by the representative location of the 4monochrome-signalcorrector 13 of FIG. 1, then the signal-precorrecting apparatus may alsoinclude circuit means for supplying the modified monochrome -signal M tothe carrier-signal encoder of the transmitter, that is, to theradio-frequency transmitter itself. Such .circuit means may include,ttor example, a suitable lter 52 and the output terminal 53 at whichappears the properly precorrected monochrome signal M.

Operation of FIG. 5.--Signal-precorrecting apparatus Considering now theoperation ot the apparatus of FIG. 5 just described, such apparatusserves to modify the conventional or present practice monochrome signalY supplied thereto in accordance with the relationship expressed byEquation 34 to produce the desired signal precorrection. Morespecifically, the input signal Y' is squared by the squaring circuit 45,translated by the modulator 4S and adding circuit 49, and thensquarerooted by the square-root circuit 46. Assuming that no othersignals are supplied to the modulator 48 and adding circuit 49, then thesuccessive squaring and squareroot operations would result in theoriginal Y signal appearing at the output terminal 53. Modification ofthe conventional Y signal does, however, occur in the modulator 48 and,additionally, due to the further components added in at the addingcircuit 49, produces the desired modication to precorrect the quadraturedistortion. Such modification is performed primarily on the squaredsignal, as represented by modulator 48, in order to take into accountthe nonlinear etects of the picture tube at the receiver.

Modification of the conventional Y' signal is obtained by generating aso-called quadrature signal which gives a measure of the quadraturedistortion which will be subsequently suiered in the receiver. Suchquadrature signal is developed by the quadrature-signal generator 41which is effective to pass the single-side band portion M of the Y videosignal and then to phase-shift these singleside-band components by thequadrature factor of 90 to develop a quadrature signal This signal isthen squared by the squaring circuit 44 to take into account thenonlinear effects of the receiver picture tube. This squared quadraturesignal is then proportioned by the signal attenuator 50 and thedirect-current restorer 51 to develop the desired a and terms as definedby Equation 33. With regard to the a term, the direct-current restorer51 is effective to establish the unity reference level to which thedesired proportion of the square quadrature signal is added as indicatedby Equation 33. The proper fraction of this signal may be obtained byproperly proportioning the attenuation in the input circuit of thedirect-current restorer 51.

- n this manner, a signal corresponding to the a term is developed whichis representative of part of the de- 12 sired moditication of the Ymonochrome signal and, hence, is supplied to the modulator 48 tomodulate or control the amplitude thereof in accordance with suchfactor. Similarly, a signal representative of the term is supplied tothe modulator 47 for modulating the un- Y squared Y signal suppliedthereto. The unsquared or linear Y term, as modified and supplied to theadding circuit 49, is necessary in order to compensate for lineardistortion components which are effectively coupled in by the envelopedetector quadrature distortion in the receiver. This so-called linearterm is then combined with the modied square term in the adding circuit49 and the resultant signal M2 is supplied to the square-root circuit 46to produce at the output thereof the precorrected monochrome signal M asdefined by Equation 34.

Description 0f FIG. 6.--SgnaI-precorrecting apparatus Referring now toFlG. 6 of the drawings, there is shown an alternative embodiment otsignal-precorrecting apparatus that may be used as the monochrome-signalcorrector 13 of the FIG. l transmitter. The parts of the FIG. 6apparatus which correspond to parts of the FIG. 5 apparatus have beenindicated by the same reference numerals. In particular, thequadrature-signal generator 41 has been shown as a single box instead ofshowing the details thereof as in FIG. 5.

Like the apparatus of FG. 5, the apparatus of FIG. 6 includes circuitmeans responsive to the quadrature signal developed by the generator 41for modifying the monochrome signal Y to precorrect the quadraturedistortion. Such means includes the squaring circuit 45, modulatorcircuit means, and a square-root circuit 60. In this case, the modulatorcircuit means includes an inverse modulator 61 and a pair of addingcircuits 62 and 63. The signal-modifying circuit means also includesmatrixing circuit means represented by the signal attenuator 50, a phaseinverter 65, a direct-current restorer 66, another inverse modulator 67,another signal attenuator 63, and a squaring circuit 69. The inversemodulators 61 and 67 may, for example, take the form of a pentodemodulator tube having a pair of input control electrodes where the tubeis operated so that the characteristic for one of the input electrodesis such that it is nonlinear in nature. This utilizes the fact that anonlinear input-output characteristic may be designed to give an outputthat is approximately the reciprocal of the input. Such reciprocal isthen multiplied with the signal supplied to the other input controlelectrode in accordance with conventional modulator theory to producethe desired inversely modulated signal at the output.

Operation of FIG. 6.-Signal-p'ecorl'ecting apparatus Considering now theoperation of the FG. 6 signalprecorrecting apparatus just described,such apparatus modifies the conventional monochrome signal Y inaccordance With the relation described by Equation 37 to produce thedesired precorrected signal M which is subsequently supplied to theradio-frequency transmitter. As before, the successive squaring andsquare-root operations performed by squaring circuit and squarerootcircuit 66 would, in the absence of other signals, result in an outputsignal corresponding to the original input signal Y. Such signal is,however, modified while in a square condition by the inverse modulator61. The factor by which it is modified or multiplied is the reciprocalof the term as dened by Equation 35. Such term, as is indicated,represents a certain proportion of the squared quadrature signal MXgenerated by the quadrature-signal generator 41. The desiredproportioning is obtained by the phase inverter 65 which serves tosupply the minus sign and the direct-current restorer. 66 which servesto supply the unity reference factor from which the negative signal issubtracted as indicated in Equation 35.

Additional quadrature factors are added in by the adding circuits 62 and63 in accordance with the correspond- Iing yterms of Equation 37. Thus,one term is added by the adding Vcircuit 62 before the square-rootoperation occurs while the other term is added by the adding circuit 63after the squaring operation has occurred, As indicated, theseadditional terms represent further proportions of the squared quadraturesignal MX and are obtained by multiplying the term by the reciprocal ofthe term in the inverse modulator 67, reducing the resultant ratio by afactor of one-half in the signal attenuator 68, and then supplying theresultant to both the adding circuit 63 and the squaring circuit 69. Thesquaring circuit 69, in turn, squares the resultant signal before it issupplied to the adding circuit 62. In this manner, the resultingprecorrected monochrome signal M appearing at the output of the circuit63 is properly modified in accordance with the relations described byEquation 37.

FIG. 7.-Wde band phase shifter Referring now to FIG. 7 ofthe drawings,there is shown an adjustable phase shifter circuit which is capable ofhandling rather wide band video signals and, hence, is particularly`suited for use as the phase shifter in the quadrature-signal generator41 of either the FIG. 5 or FIG. 6 apparatus. This wide band phaseshifter of FIG. 7 operates on the fact that single-side-bandcarrier-modulation components are continually varying in phase and uponthe fact that synchronous detection of such signal components may beutilized to extract such components at any desired phase angle.

More particularly, as shown in the representative embodiment of FIG. 7,-a carrier signal of suitable carrier frequency is generated by acarrier-signal generator 7d and then supplied `to a modulator 71. Alsosupplied to the modulator 71 is an input signal which represents thevideo signal which is to be phase-shifted. The modulator 71 is etiectiveto encode theinput signal as amplitude modulation of the carrier signalin a conventional manner. The resulting carrier signal which is doubleside band in nature is then supplied to a single-side-band filter 72which suppresses one of the sets of side bands thereby producingasingle-side-band signal at the output thereof. In this manner, theinput signal is converted to single-sideband modulation componentswhich, because they are single side band in nature, are continuallyvarying in phase relative to the carrier phase.

Such single-side-band components are then supplied to a synchronousdetector 73. Also supplied to the synchronous detector 73 is the carriersignal developed by the carrier-signal generator 7) vafter beingtranslated by a phase shifter 74. The carrier signal reaching thesynchronous detector 73 by way of the phase shifter 74 is unmodulated innature, that is, of constant amplitude but of the same frequency as thecarrier signal upon which the input signal is encoded. In accordancewith conventional synchronous detector theory, such unmodulated carriersignal heterodynes with the single-side-band components to reduce themdirectly to video frequency, that is, to detect them. In other words,the difference-frequency components arising from the heterodyning actionare of video frequency the same as if the modulated car- Iier signal`had been detected by a simple diode detector.

It is an important characteristic of the synchronous detector 73,however, that the phase of the video output signal from the detector 73corresponds to the phase of the unmodulated constant amplitude carriersignal supplied to the synchronous detector 73 by way of the phaseshifter 74. In other words, synchronous detection of single-side-bandcomponents changes the phase but not the amplitude of such components.Thus, the desired phase shift that is produced is the phase shift whichis given to the unmodulated carrier signal as it passes through thephase shifter 74. This phase shift may be selected to be any desiredvalue.

The important point is that the phase shifter 74 need not be in any wayWide band in nature and, in fact, the

band width thereof might be very narrow.. This is because the phaseshifter 74 only has to translate a single frequency component, namely,the unmodulated carrier signal. The band width of the input videosignal, on the other hand, is determined by the band widths of themodulator 71, the single-side-band filter 72, and the synchronousdetector 73 and, hence, these units must have band Widths correspondingto the band width of the input video signal. It is well known, however,how to build these units` with relatively wide band widths. Accordingly,the phase-shifting apparatus of FIG.4 7 represents a new and improvedway of obtaining the desired phase shift of a relatively wide band videosignal.

CONCLUSION From the foregoing descriptions of the Various embodiments ofthe invention it will be apparent that signalprecorrecting apparatusconstructed in accordance with the present invention represents new andimproved apparatus for use in the transmitter of a vestigial-side-bandcommunications system for precorrecting envelope detector quadraturedistortion which would otherwise occur in the receiver.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be rnadetherein without departing from the invention, and it is, therefore,aimed to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. A communications system comprising: a transmitter for transmitting anamplitude-modulated carrier signal including an intelligence-signalcomponent, part of which is transmitted in a single-side-band manner; areceiver including an envelope detector for detecting theintelligencesignal component, such envelope detection of thesingleside-'band portion of the intelligence-signal component causingquadrature distortion of the detected signal; and means included in thetransmitter comprising circuit rmeans for deriving a signalrepresentative of said quadrature distortion, and circuit meansresponsive to said representative signal for modifying the intelligencesignal prior to encoding on the carrier signal, to precorrect for suchquadrature distortion.

2. A communications system comprising: a transmitter `for transmittingan amplitude-modulated carrier signal including an intelligence-signalcomponent, part of which is transmitted in a single-side-band manner; areceiver including `an envelope detector for detecting theintelligence-signal component, such envelope detection of thesingle-side-band portion of the intelligence-signal component causingquadrature distortion of the detected signal; and means including in thetransmitter comprising nonlinear circuit maens responsive to theintelligence signal for generating a quadrature signal representative ofsaid quadrature distortion and nonlinear circuit means responsive tosaid quadrature signal for modifying the intelligence signal prior toencoding on the carrier signal to precorrect for such quadraturedistortion.

3. A television system comprising: a transmitter for transmitting anamplitude-modulated carrier signal including at least amonochrome-signal component, part of which is transmitted in asingle-side-band manner; a receiver including an envelope detector fordetecting the monochrome component, such envelope detection of thesingle-side-band portion of the monochrome component causing quadraturedistortion of the detected signal; and means included in the transmittercomprising nonlinear circuit means responsive to the monochrome signalfor generating a quadrature signal representative of said quadraturedistortion and nonlinear circuit means responsive to said quadraturesignal for modifying the single-sideband portion as well as thedouble-side-band portion of the monochrome signal prior to encoding onthe carenligne? rier signal to precorrect for such quadraturedistortion.

4. A television system comprising: a transmitter for transmitting anamplitude-modulated carrier signal including at least amonochrome-signal component, -part of which is transmitted in asingle-side-band manner; a receiver including an envelope detector fordetecting the to precorrect for such quadrature distortion.

5. A television system comprising: a transmitter for transmitting anamplitude-modulated carrier signal including at least amonochrome-signal component, part of which is transmitted in asingle-side-band manner; a receiver including an envelope detector fordetecting the monochrome component, a monochrome-signal channel oflimited band width coupled to the detector for translating themonochrome signal, and a nonlinear image-reproducing device coupled tothe monochrome channel for reproducing the televised image, suchenvelope detection of the single-side-band portion of the monochromecomponent causing quadrature distortion of the detected signal which issubsequently modified by the band-widthlimited monochrome channel andnonlinear image-reproducing device and produces spurious effects in thereproduced image; and means included in the transmitter comprisingcircuit means, having a nonlinearity corresponding to the nonlinearityof the receiver image-reproducing device and responsive to themonochrome signal, for generating a nonlinear quadrature signalrepresentative of envelope detector quadrature distortion afternonlinear processing thereof, and circuit means responsive to thequadrature signal for modifying the single-side-band portion as Well asthe double-side-band portion of the monochrome signal prior to encodingon the carrier signal to precorrect for such quadrature distortion inthe receiver.

6. Sginal precorrecting apparatus for use in the transmitter of acommunications system where at least a part of an intelligence signal istransmitted in a single-sideband manner, the apparatus comprising:

circuit means for supplying an intelligence signal;

circuit means for deriving a signal representative of envelope detectorquadrature distortion;

and circuit means responsive to said representative signal for modifyingthe intelligence signal prior to encoding on a carrier signal toprecorrect for quadrature distortion caused by envelope detection of thesingle-side-band portion of the transmitted intelligence signal in areceiver.

7. Signal-precorrecting apparatus for use in the transmitter of acommunications system Where at least a part of an intelligence signal istransmitted in a single-sideband manner, the apparatus comprising:circuit means for supplying an intelligence signal; circuit meansresponsive to the intelligence signal for generating a quadrature signalrepresentative of envelope detector quadrature distortion; and circuitmeans responsive to the quadrature signal for modifying the intelligencesignal prior to encoding on a carrier signal to precorrect forquadrature distortion caused by envelope detection of thesingle-sideband portion of the transmitted intelligence signal in areceiver.

8. Signal-precorrecting apparatus for use in the transmitter of acommunications system Where at least a part of an intelligence signal istransmitted in a single-sideband manner, the apparatus comprising:circuit means for supplying an intelligence signal; nonlinear circuitmeans responsive to the intelligence signal for generating a quadraturesignal representative of envelope detector quadrature distortion; andnonlinear circuit means responsive to the quadrature signal formodifying these signalside-band portion of the intelligence signal priorto encoding on a carrier signal to precorrect for quadrature distortioncaused by envelope detection of the single-sideband portion of thetransmitted intelligence signal in a receiver.

9. Signal-precorrecting apparatus for use in a television transmitterwhere at least a monochrome-signal component is partially transmitted ina single-side-band manner, the apparatus comprising: circuit means forsupplying a monochrome signal; circuit means responsive to themonochrome signal for generating a quadrature signal representative ofenvelope detector quadrature distortion; and circuit means responsive tothe quadrature signal for modifying the single-side-band portion of themonochrome signal prior to encoding on a carrier signal to precorrectfor quadrature distortion caused by envelope detection of thesingle-side-band portion of the transmitted monochrome signal in areceiver.

10. Signal-precorrecting apparatus for use in a television system Whereat least a monochrome-signal component is partially transmitted in asingle-side-band manner and the receiver includes a nonlinearimage-reproducing device, the apparatus comprising: circuit means forsupplying a monochrome video signal; nonlinear circuit means, having anonlinearity corresponding to the nonlinearity of the receiverimage-reproducing device and responsive to the single-side-band portionof the monochrome video signal, for generating a nonlinear quadraturesignal representative of envelope detector quadrature distortion afternonlinear processing thereof; nonlinear circuit means responsive to thequadrature signal for modifying the single-side-band portion of themonochrome video signal to precorrect for quadrature distortion causedby envelope detection of the single-side-band portion of the transmittedmonochrome signal in a recever and circuit means for supplying themodified monochrome video signal to the carrier-signal encoder of thetransmitter.

11. Signal-precorrecting apparatus for use in a television transmitterwhere at least a monochrome-signal component is partially transmitted ina single-side-band manner, the apparatus comprising: circuit means forsupplying a monochrome video signal; a rst signal channel coupled to thesupply circuit means and including a squaring circuit, modulator circuitmeans, and a squareroot circuit for translating the monochrome videosignal; a second signal channel coupled to the supply circuit means andincluding a band-width-limiting lter, a phase shifter, and a squaringcircuit for generating a quadrature signal representative of envelopedetector quadrature distortion; matrixing circuit means forproportioning the quadrature signal and for supplying proportionedquadrature-signal components to the modulator circuit means of the firstsignal channel for modifying the single-band portion of the monochromevideo signal to precorrect for quadrature distortion caused by envelopedetection of the single-side-band portion of the transmitted monochromesignal in a receiver; and circuit means for supplying the modiedmonochrome video signal to the carriersignal encoder of the transmitter.

References Cited lay the Examiner UNITED STATES PATENTS 2,552,588 5/1951Reeves l78-7.l X 2,668,238 2/1954 Frink 328--155 2,717,956 9/1955 Elgin325-65 2,777,900 1/1957 Cowan 332-37 X 2,798,201 7/1957 Moulton et'al.332-1 2,849,537 8/1958 Eglin 325-65 3,011,018 1l/l961 Sullivan 178-6DAVID G. REDINBAUGH, Primary Examiner.

ROBERT SEGAL, Examiner.

1. A COMMUNICATIONS SYSTEM COMPRISING: A TRANSMITTER FOR TRANSMITTING ANAMPLITUDE-MODULATED CARRIER SIGNAL INCLUDING AN INTELLIGENCE-SIGNALCOMPONENT, PART OF WHICH IS TRANSMITTED IN A SINGLE-SIDE-BAND MANNER; ARECEIVER INCLUDING AN ENVELOPE DETECTOR FOR DETECTING THE INTELLIGENCESIGNAL COMPONENT, SUCH ENVELOPE DETECTION OF THE SINGLESIDE-BAND PORTIONOF THE INTELLIGENCE-SIGNAL COMPONENT CAUSING QUADRATURE DISTORTION OFTHE DETECTED SIGNAL; AND